Because of environmental concerns, there are increasing efforts to reduce polluting emissions and achieve fuel savings in vehicles. The nature of the engine lubricant has an influence on these two phenomena. The behaviour of the lubricant in the reduction of friction has an impact on fuel consumption. It is mainly the quality of the base oils, alone or in combination with polymers improving the viscosity index (VI) and friction modifying (FM) additives, that gives the lubricant its “Fuel Eco” (FE) properties.
Among the friction modifiers used conventionally, molybdenum dialkyldithiocarbamate (or Mo-DTC), which is an inorganic friction modifier, and glycerol monooleate (or GMO), which is an organic friction modifier may be mentioned. GMO, in contrast to Mo-DTC, is a friction modifier which has the advantages that it does not contain ash, phosphorus or sulphur and is produced from raw materials of renewable origin. However, its friction properties are not as good as those of Mo-DTC, for example. It would therefore be desirable to be able to formulate lubricant compositions with friction modifiers obtained from compounds originating from raw materials of renewable origin, not comprising ash, having improved friction properties relative to lubricant compositions comprising GMO.
The use of a mixture of esters for reducing friction in engines is known from U.S. Patent Publication No. 2005/075254 and PCT Patent Publication No. WO2005/030912. These esters are obtained from an esterification reaction between a polyol and a cyclic fatty acid comprising from 12 to 28 carbon atoms and/or a branched fatty acid comprising from 12 to 28 carbon atoms. U.S. Patent Publication No. 2008/176778 describes a lubricant composition in the form of an emulsion. This composition in particular comprises a lipophilic compound and an emulsifier. The lipophilic compound can in particular be selected from caprylate or caprate triglycerides and the emulsifying compound can in particular be glycerol monooleate.
U.S. Pat. No. 5,064,546 describes a lubricant composition comprising a base oil and a friction modifier, which can be a coconut oil. The document JP2005082709 describes a method for reducing friction in engines using a lubricant composition comprising at least an glycerol ester. These esters are obtained from carboxylic acids comprising from 10 to 18 carbon atoms. U.S. Patent Publication No. 2005/070450 describes lubricant compositions comprising a product obtained by a transesterification reaction between an ester of glycerol and an ester of polyol different from glycerol.
The present invention therefore relates to lubricant compositions in particular for engines, comprising at least two glycerol esters with different chemical natures. The combination of these two glycerol esters of different chemical natures makes it possible to obtain good performance in terms of friction and in terms of fuel economy.